linux-yocto/4.14: fix kernel configuration audit warnings

linux-yocto/4.14: fix kernel configuration audit warnings

Some of the reference BSPs for 4.14 need fixes that have been done
on 4.18 and master to silence warnings, so we cherry-pick the
following changes:

  f991b21d7858 config: flash: drop obselete config values
  da7524cf2980 edgerouter: Drop the obsolete kernel options
  675c5b0cde65 cfg: drop MACH_VERSATILE_PB and MACH_VERSTAILE_AB
  0936b18ed8d5 cfg/timer/rtc: toggle RTC_CLASS instead of GEN_RTC
  634d8f2230d1 mpc8315e-rdb: Drop the obsolete kernel options
  9beb3a28736e rt: drop obselete configuration options

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>